Abstract

Lunar bases require 2m of regolith shielding (Kaplicky & Nixon, 1985) and up to 4m during intense solar flare activity (Silberberg et al, 1985). This severely impacts the extent and costs of lunar surface base development. Greeley & Spudis (1986) and others have described apparent giant lava tubes in roofed-over portions of lunar sinuous rilles. Horz (1985) calculates the intact roof sections have 10m or more overburden and recommends giant lava tube interiors as natural shelters for lunar bases. We conducted educational lunar lava tube base simulations in cooperation with Young Astronauts, Students for the Exploration and Development of Space (SEDS), Oregon Museum of Science and Industry (OMSI), U.S. Forest Service, and the City of Bend (OR). Terrestrial lava tubes made convenient, cost-effective shelter and were valuable for testing lunar base parts, plans, and procedures. Our facility will be available for researchers and for educational simulations.

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Lunar Bases of the 21st Century

There is steadily increasing interest in lunar bases and a realization that such bases will enable humanity to utilize the rich resources available on and in the Moon (see for example Mendell, 1985). These resources include oxygen (as rock oxides), silicon, aluminum, iron, titanium, magnesium, calcium, and a variety of trace rare earth elements. Although the Moon is depleted in volatiles (water, lighter elements), it is now thought that lunar regolith (ultra-fine rock flour, covering the surface to a depth of 10m) has embedded in it some light elements (3He, for example) from the eons of bombardment by the solar wind (Cameron, 1988; Crabb & Jacobs, 1988; Gibson, 1988; Haskin, 1988; Santarius, 1988). As a result the mission of lunar bases has been expanded from scientific research to include mining of lunar resources and manufacturing of lunar-derived materials. The bases have grown in conception from cramped cylinders protected by piling regolith over them to industrial parks and residential facilities, with expanded equipment and life-support needs, located under broad sheltering structures and integrated with orbiting facilities to enable a truly spacefaring civilization (Bova, 1987; Second Symposium on Lunar Bases and Space Activities of the 21st Century, 1988).

According to Silberberg et al. (1985), "The cosmic-ray environment on the lunar surface is inhospitable for permanent settlement. There is no radiation-absorbing atmosphere and no overall magnetic field that deflects charged particles. Also, the lunar surface is not protected from solar flare particles. For permanent lunar residents, it is necessary to construct shelters several meters below the lunar surface." Some researchers have proposed living in tunnels left by lunar mining excavators (Graham, 1988; Rocha and Carr, 1988). Other authors have envisioned the needs for "radiation storm cellars" to protect otherwise exposed personnel from sudden unexpected solar flares (Land, 1985; Angelo et al ., 1988).

The usual method for achieving minimal protection from these radiation hazards is to pile 4m of commonly available lunar regolith over the structure or process to be protected (Christiansen et al ., 1988). This requirement imposes severe restraints on the designs and areal extents of lunar surface bases. Crockford (1988) notes, "placement of clear spans which are both protective and of large areal extent is not simple." Ben Bova, in his book Welcome to Moonbase (1987), estimates "construction engineers and swarms of teleoperated machines" would labor for "three years" using "microwave beams and plasma torches" to construct a shielded vault 75m high and 600m long.

We may be able to find natural features on the Moon which would offer equivalent or better protection without spending years of time, labor, risk, and expense in construction. While lunar base researchers were investigating methods of mining, manufacturing, and utilizing lunar resources, lunar geologists continued their study of Apollo and other data. In the course of that research, the existence of lunar lava tube caverns has been strongly suggested (Cruikshank & Wood, 1972; Greeley & Spudis, 1986; Oberbeck et al ., 1969), so that now Spudis, Swann, and Greeley (1988) can claim, "The origin of lunar sinuous rilles via the lava channel/tube mechanism is accepted by most investigators."

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Lunar Lava Tubes

In the final stages of lunar formation, lasting from about 3.8 to 3.0 billion years ago, voluminous flows of very hot, fluid lava poured from fractures associated with basin-forming impacts, resulting in the extensive lava fields known as lunar maria (Solomon, 1986). Numerous lunar features known as "sinuous rilles" are found in association with these lava flows. The question of rille origin and emplacement was important enough to send the Apollo 15 mission to investigate the region of Hadley Rille, one of the best-known of these features (Spudis & Ryder, 1986). Analysis of the Apollo 15 data confirmed that "Hadley Rille appears to be a collapsed lava tube/channel...." (Greeley & Spudis, 1986). According to Greeley (1971), "[Lava] Tubes are so common in pahoehoe flows that they are evidently the primary means of flow advance." ('Pahoehoe' denotes a low-viscosity, high-temperature type of lava which also describes the lunar maria flood basalts.)

Oberbeck et al . (1969) calculated that basalt "bridges" or roofspans 500m wide were possible given the Moon's light gravity, with a basalt thickness of 40-60m overhead. Data from impact cratering sustained by intact roofs of probable lava tubes also suggests a thickness of at least 40m (Horz, 1985). As Horz (1985) points out, "it appears that natural caverns of suitable sizes to house an entire lunar base exist on the Moon. Roof thicknesses in excess of 10m will provide safe and long-term shelter against radiation and meteorite collisions." In addition, the thermal stability of these natural caverns is expected to result in a constant temperature of -20°C (-4°F) (Mendell in Horz, 1985), compared to the temperature flux approaching 300°C (540°F) on the surface.

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Lava Tube Morphology

Lava tubes form naturally from the cooling edges and surface of a low-viscosity basalt flow, eventually encasing the moving stream of fluid rock. At the end of an eruption, the basalt river drains from its conduit leaving behind a sinuous cave that retains many of the forms characteristic of a molten flow. Shear planes in the flow sometimes cause multiple tubes to be stacked one upon another, or result in the formation of benches or bridges within a single tube (Greeley, 1971). Skull Cave in Lava Beds National Monument of North California and Lava River Cave near Bend, Oregon contain these features.

Sometimes the dynamics of the lava flow forms a ceiling channel with a spindle-shaped cross-section called a cupola. Occasionally pressures may cause the lava river to breach the ceiling and become a kind of "false vent" on the surface. Generally, though, the ceilings are arch-shaped.

Smaller flows as the eruption ceases, or subsequent eruptions, may erode the floor into a smaller channel within the tube. Derrick Cave in south-central Oregon demonstrates this feature, and the secondary flow even forms a small "tube within a tube" (Harter and Harter III, 1982). (On the Moon we can find sinuous rilles with smaller channels meandering down the center of the rille.)

Cross-sectional shape of the tube also is dependent on the slope of the underlying surface over which the tube is emplaced. Steep slopes (>1°) produce either nearly-circular profiles, shown in Skeleton Cave near Bend, or tall narrow tubes such as Wind Cave near Bend or Ape Cave on the slopes of Mt. St. Helens, Washington. Lavas pouring over nearly-level landforms (<<1°) tend to spread more and form low, broad caves, sometimes a number of interconnected, parallel "anastomosing" caves such as the Horse system in Bend or Valentine Cave at Lava Beds National Monument.

Upon drainage of the formational lavas, the support for the roof is withdrawn and the rock walls slowly cool. Much of the collapse that is found in lava tube caves happens at this time (Harter and Harter III, 1982), resulting in rockpiles with large, angular boulders piled, sometimes many meters deep and at slopes up to 22°, on the cave floor (Greeley, 1971). Contra-lateral cracks often appear during the cooling stages, some of which reach the surface and allow surface sands and even small bones, etc., to wash down into the cave. We can expect such cracks in lunar lava tubes, through which some regolith may be emplaced in the cavern; however, the extensive sand-filling of some terrestrial caves is attributed to water action (Greeley, 1971) and this mechanism was never available on the Moon (Hartmann et al ., 1986).

Uncollapsed lava tube segments tend to be very strong, due to the arched structure and laminated nature of lavas left by surges and flows. In some cases further strengthening occurs when subsequent surface flows emplace additional layers of rock over the roof (Harter and Harter III, 1982). For example, Highway 97 south of Bend crosses over Lava River Cave with no apparent impact on the cave or the highway. According to Harter and Harter III (1982), "Once a lava tube has withstood the stress of cooling shrinkage, it is ordinarily very stable."

The mechanical properties of the highly fluid basalt which formed terrestrial lava tubes is not dissimilar to the properties of the hot lunar basalts that flooded the maria, so that formational features we see on the Earth are likely to be found on the Moon. Observation, supported by research and calculations, indicates that the geologic form of lava tubes is found in both places, but that due to both the high volume of lunar basalt and gravity 1/6 that of Earth, the scale of these features on the Moon is 10 to 20 times that of their terrestrial counterparts. Except for size, the forms of terrestrial lava tubes are like those of the Moon.

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Utilization of Lunar Lava Tubes

The amount of excavation required to expose the entrance to a lunar lava tube is comparable to the amount of excavation required for the covering of a simple lunar research base and could be accomplished over the course of a single mission. In return, a ready-made volume is accessed that may be 500m wide, hundreds of meters high and many hundreds of meters or even kilometers long. At least initially, this will be too gigantic to consider sealing the cave itself even if we could afford to bring enough air to fill it. Since the natural vacuum of the Moon may be of value to certain industrial processes and for research purposes, the cavern will be left unpressurized. But there is enough room in these caverns to emplace several prefabricated modules.

Since habitats emplaced in lava tubes do not need to support weighty regolith or other shielding, they need only be made strong enough to contain an atmosphere. Extremely lightweight, inflatable structures are possible under such conditions (Roberts, 1988). Structures need not adhere to the size limitations of delivering spacecraft nor the shape requirements dictated by the need to support shielding. Windows could be a part of these constructions, which would look out on the vast interior spaces of the lava tube (Daga et al ., 1988).

Life support systems and industrial processes that on the surface require protection can be left out in the open. In addition, the mechanical components that would have to be jammed together for efficient use of shielding can be spread apart for ease of construction and maintenance (Horz, 1985).

Workers outside the habitats but still inside the lava tube would be able to wear lightweight, minimal space suits, perhaps a 'second skin' type design, which will make activities much easier (Horz, 1985).

The vast cave can also be used as a storage depot for supplies and products that need to be protected from the surface conditions, and as a garage for equipment used on the surface. The interior of lunar lava tubes maintains a nearly-constant temperature (as do those on Earth) reducing thermal stress on equipment and personnel.

The unique conditions of lava tubes may be utilized in design and construction. For example, the naturally arched roof may serve to suspend base elements and/or transportation systems over the rockpiles we expect to find littering the floors (Horz, 1985). The rockpiles themselves may be raw material for walls, platforms, and a variety of natural stone constructions (Khalili, 1988). Interior channels, although small in comparison to the parent cave, may be large by terrestrial standards—large enough to roof-over and pressurize to form habitable areas or even parks. If sealing these channels is impractical, their parallel rock walls may still provide bracing to support base elements, transportation systems, utility services, etc. The strong arched roof can be purposely pierced in order to give service access to/from the surface: large freight and fast passenger elevators could be emplaced; sunlight could be concentrated and projected into interior areas; and power cables and signal lines could take the shortest route from remote reactor power sources or data devices to the base. Finally, the gentle gradient of the tube interior might be of use for certain industry and service needs.

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Simulations for Testing, Training, and Educating

The Pacific Northwest region is rich in lava tubes, some of which are quite large by terrestrial standards — 12-40m wide and high. These natural features serve as large-scale models to investigate problems and advantages of lunar lava tubes.

The value of simulations for training and testing is widely recognized. Simulations permit personnel and equipment to be tested in realistic, interactive situations. Simulations take many forms, from underwater space simulations to instrumented rocking cockpits to incorporeal computer models. Lunar base simulation in a natural lava tube allows researchers to field-test designs and procedures, to take advantage of natural features not available in the laboratory, and to discover benefits and problems unique to this environment.

Oregon L-5 Society sponsors three Young Astronaut chapters at the Oregon Museum of Science and Industry (OMSI) in Portland, Oregon. Researchers and chapter leaders Bryce Walden, Thomas Billings, and Cheryl Singer developed lunar base simulations to teach sciences, organization, and teamwork. At the same time, these simulations could be of value in the development of real lunar bases, contributing to the discussion of lunar lava tube bases.

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Lunar Base Design and Site Selection

OMSI-L5 Young Astronauts in grades 4-9 and OMSI Students for the Exploration and Development of Space (SEDS), from high school, were cast in the role of lunar base personnel. Researchers helped the Young Astronauts design lunar base functional modules using the plastic tube-and-connector "QUADROS" construction system, supplied by OMSI's Pacific Rim Space Education Center (PaRSEC). The basic lunar outpost consisted of four functional modules: a sleep/work platform, a galley, a sanitary facility, and a power/communications console (for safety, 12v power was standard). A timeline was constructed and the mission profile defined. Meanwhile, a search was initiated to find a suitable lava tube site.

A survey of Pacific Northwest lava tubes by the researchers resulted in a list of large lava tubes containing features like those which might be found on the Moon (see Appendix). For initial educational simulations, a cave with easy access and a wide, flat floor was selected: Skeleton Cave near Bend, Oregon (named after bones of extinct Pleistocene animals found within). This is a popular recreational lava tube, roughly 12m in diameter and about 1km long. The U.S. Forest Service graciously granted us exclusive-use permits for up to four weekend-long mission simulations during the 1987-88 school term. In this cave, it was not necessary to clear the entrance or level the floor. Although entrances will undoubtedly need to be cleared to reach lava tubes on the Moon, interior portions of these tubes may indeed have flat floors with no rockfall, where cooling ponds of lava have solidified. This feature is not unusual in terrestrial lava tubes.

A preliminary feasibility study was conducted in Skeleton Cave by chapter leaders before bringing Young Astronauts and SEDS personnel on a full mission. Minor adjustments were made to plans based on this experience; otherwise, all indicators were go for the mission.

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Mission Outline

Friday after school participants rendezvoused at OMSI to load cargo vehicles and board passenger shuttles. These "spacecraft" were "launched" from the OMSI parking lot (representing Earth) and travelled over the Cascade mountains (representing the long flight from the Earth to the Moon) to the mission site in Bend's "Moon Country," where astronauts were trained by NASA in the 1960's. On some missions, Sunriver Preparatory School near Bend served as a "space station" overnight on Friday night.

On Saturday base personnel donned hardhats and conducted a familiarization and litter sweep through the cave. Personnel follow cave safety and conservation practices recommended by the National Speleological Society. A specific site was chosen within the lava tube cave and base elements were deployed. After base construction, participants prepared supper at the base galley, deep underground. On Saturday night the clear eastern Oregon skies gave ample opportunity for telescopic examination and constellation identification. Participants then spread their sleeping bags on the sleep platform--no more shelter was necessary--and spent the night in the cave.

The next morning, mission science activities were conducted and results compiled. Then, after lunch in the cave, the base was dismantled and stowed in the cargo vehicles and participants left for the long "flight" back to "Earth" at OMSI. A "debriefing" provided researchers with feedback and allowed personnel to review the mission. (A more complete description of the base simulation and mission science activities is given in the companion paper, "Educational Simulations of Lunar Bases Inside Lava Tubes," Walden, 1988a).

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Results of Lava Tube Lunar Base Simulations

The missions were rated a success by researchers and participants. There was great enthusiasm for the lunar base simulation as an educational experience. Designs for equipment and work stations were tested in a rough, dusty, hard-rock, lunar-like working environment. That environment itself suggested new designs and opportunities, some of which were mentioned above. Personnel were tested, too, by working in teams and conducting science activities in the field. Overall working and activity patterns were easily studied in the realistic environment.

These mission simulations conducted in lava tubes demonstrated the potential for savings on actual lunar base designs. Capital costs including the full value of parts loaned by OMSI were less than $5,000. It was not necessary to go to the time, expense, and labor of erecting a protective shell and piling dirt on top of it. Using the natural shelter of the lava tube allowed the base to become fully functional much earlier in the mission. This increased the payback in mission science and activities.

The lava tubes of Bend did hold one surprise: they are full of dust. Fine clouds of it puff up under each footstep, and hang in the air. It clings to people, clothes, and equipment with great tenacity, interferes with the fit of close-tolerance parts, and contaminates lubricants. Particularly in work areas, it would pervade the atmosphere and become a respiratory irritant. The immediate solution was to cover the sandy cave floor with tarpaulins, to keep the dust from being stirred up at the center of activity. More complete designs will include active and passive measures to reduce dust contamination. This fine dust is relatively unique to Bend area lava tubes, being volcanic 'ash' from the eruption that created Crater Lake almost 7000 years ago (Chitwood, 1982). Microscopic examination reveals myriads of these abrasive, glassy dust particles coating every grain of cave sand. This volcanic dust is a very good mechanical analog to the fine dusty powder of lunar regolith which is expected to be just as problematical to lunar settlers. This adds to the realism of Bend area lava tubes for lunar lava tube base simulations.

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A Permanent Lava Tube Basefor Education and Research

These lunar base missions were restricted by the need to reconstruct the base for each mission and to take time to tear it down at the end. Long-term progress cannot be made under such conditions, so the researchers opened the search for a permanent site. A permanent site would allow lunar base infrastructure to build up over time and permit each mission to conduct more science and related activities. Complex projects could expand and develop. Researchers could test equipment and procedures without themselves having to duplicate and manage an entire lunar base. A permanent site also would provide a standard testbed for comparison purposes between competing designs or systems.

Two sites were targeted for future development. The eastern portion of Lava River Cave is boarded-up and closed to the public. We acquired permission to investigate this cave and found it to be very large with a floor buried under mounds of large (1 meter) angular basalt blocks. This is the condition we expect to find in lunar lava tubes, but it was deemed too rough for the current level of educational simulations. It would be an ideal facility to develop as a top-level, industrial lunar base simulation.

The other site is Young's Cave, closed to the public but located on land owned by the City of Bend. Portions of this lava tube, which is actually a pair of major lava tubes with a small joining passage, are tall and broad. Other sections are wide but low. Where Lava River Cave developed on a steep slope, Young's Cave is on nearly-level ground and displays the anastomosing or interlaced structure of this kind of flow. The cave is excellent for educational simulations and has several areas suitable for deployment of full-scale lunar base components. Some entrances nearly or completely blocked by rockfall and sand provide opportunities to test lunar regolith-moving designs, adjusted for Earth's heavier gravity.

A mission simulation was conducted at Young's Cave with permission of Bend city officials, and the site appears to be suitable. We approached the City Council with a proposal to use Young's Cave for further work. The city greeted our proposal with enthusiasm and has offered us use of the lava tube complex for educational and research purposes. We intend to conduct educational simulations and build up a base infrastructure as soon as the caves can be made secure. It is hoped that industry and academia will support this facility both as an educational project and as a proving-ground for lunar designs and procedures. Cooperative arrangements may be made with researchers, schools, and universities for projects to be built and field-tested. Simulations may be designed and run at different levels of complexity, from relatively simple simulations suitable for grade-school youth to complex full scale lunar simulations to train actual Moonbase personnel (for further discussion of a 15-year development plan, see Billings, Walden, and Dabrowski, 1988). Educators and researchers interested in the potentials of an operating lava tube lunar base are cordially invited to contact the author.

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References

Angelo, Joseph A. Jr., Madonna, Richard G., and Quam, William (1988), Radiation Protection Issues and Techniques in Support of Lunar Base Operations, in Abstracts of the Second Lunar Bases and Space Activities of the 21st Century Conference, Lunar and Planetary Institute, Houston.

Cameron, E.N. (1988), Mining for Helium - Site Selection and Evaluation, in Abstracts of the Second Lunar Bases and Space Activities of the 21st Century Conference, Lunar and Planetary Institute, Houston.

Crabb, T.M., and Jacobs, M.K. (1988), Synergism of 3He Acquisition with Lunar Base Evolution, in Abstracts of the Second Lunar Bases and Space Activities of the 21st Century Conference, Lunar and Planetary Institute, Houston.

Crawford, W.W. (1988), Lunar Surface System Concepts - Initial Lunar Base Philosophy, in Abstracts of the Second Lunar Bases and Space Activities of the 21st Century Conference, Lunar and Planetary Institute, Houston.

Daga, Andrew W., Towarnicki, Meryl A., Wendel, Wendel R. (1988), Evolving Concepts of Lunar Architecture: The Potential of Subselene Development, in Abstracts of the Second Lunar Bases and Space Activities of the 21st Century Conference, Lunar and Planetary Institute, Houston.

Gibson, Everett K. (1988), Lunar Hydrogen: A Resource for Future Use at Lunar Bases and Space Activities, in Abstracts of the Second Lunar Bases and Space Activities of the 21st Century Conference, Lunar and Planetary Institute, Houston.

Graham, Eric K. (1988), Subselenian Mining and Habitation, in Abstracts of the Second Lunar Bases and Space Activities of the 21st Century Conference, Lunar and Planetary Institute, Houston.

Rocha, C.J., and Carr, C. (1988), Thermal Tunneled First Stage Lunar Base, in Abstracts of the Second Lunar Bases and Space Activities of the 21st Century Conference, Lunar and Planetary Institute, Houston.

Santarius, John F. (1988), Lunar 3He, Fusion Propulsion, and Space Development, in Abstracts of the Second Lunar Bases and Space Activities of the 21st Century Conference, Lunar and Planetary Institute, Houston.